1. FIELD OF THE INVENTION
The invention relates to an electric discharge machine, more particularly to one having a movable ram assembly carrying a vertically movable shaft to directly hold an electrode head, said shaft being electrically insulated from the casing of the ram assembly.
2. DESCRIPTION OF THE RELATED ART
Electric discharge (or electric-spark) machining is a metal-removal process in which materials that conduct electricity are removed by electric sparks. This process is used to form holes of varied shape in materials of poor machinability and to form cavities in steel dies. Machining of a stationary workpiece is accomplished without any grinding or cutting operation so that the workpiece is not subjected to external forces which may cause undesired deformation thereto. EDMs possess a high degree of precision and can be used for relatively long periods. These machines are preferred if accurate machining is desired.
FIG. 1 is an illustration of a conventional electric discharge machine. It mainly comprises a main machine assembly (1), a support stand (2), a movable ram assembly (3) having a vertical shaft therein (not depicted) and mounted on the support stand (2), an electrode head (4) carried by the movable ram assembly (3) and a casing (5) for safe protection of a metal workpiece provided immediately under the electrode head (4) and held in a stationary position by a workbench on the main machine assembly (1).
As illustrated in FIG. 2, the movable ram assembly (3) is drivingly coupled to a servomechanism (not shown) and a tool electrode (not shown) is attached to the electrode head (4) coaxially with the ram assembly (3) via an insulating plate (4a). Thus the electrode head (4) is electrically insulated from the movable ram assembly of the electric discharge machine.
The above-mentioned electric discharge machine has the following drawbacks:
(1) Since an electric discharge machine is a high precision machine, the mounting of said insulating
plate (4a) must be very accurate and precise. The deviation in the alignment of the axis of the vertical shaft of the ram assembly (3) and the axis of the shaft (not shown) of the electrode head (4) would significantly affect the accuracy in machining a workpiece. Since the ram assembly (3) and the electrode head (4) are electrically insulated by a flat insulating plate (4a), it is difficult to align the axes of the shaft of the electrode head (4) with that of the shaft of the ram assembly (3).
(2) The insulating plate (4a) is attached to the casing of the ram assembly (3) by means of screws (4d) through openings in the insulating plate. The openings must be exactly equal to the size of the screw (4d), any clearance left in the opening can cause error in the alignment of the ram assembly (3) and the electrode head (4).
(3) Frequent adjustment is necessary to correct the position of the electrode head (4).